Certain car radio devices, known from practice, are based on the so-called 2-IC technology. In these car radio devices, two or three freely programmable audio filters are integrated into the signal path. These digital parametric equalizers (DPE) are available to the user to compensate for acoustical shortcomings in the passenger compartment. The user is able to vary each filter with respect to center frequency, quality, i.e. filter width, and amplification or attenuation, in order to compensate for excessive rises and so-called holes in the acoustical frequency response of the passenger compartment.
However, this proves to be problematic in practice, since the user must know the acoustics of his/her vehicle very well to optimally adjust the equalizers, and it is very difficult to ascertain the acoustical frequency response solely by listening, without metrological aid. The operating instructions of the known car radio devices are only able to provide very limited assistance for the best possible adjustment of the equalizers, since on no account is it possible to consider all types of vehicles here, and by no means the great number of individual layout variants, as well as loudspeaker and amplifier configurations.
Moreover, car radio devices are known having an audio module, integrated in the signal path, on which a graphic equalizer is implemented with the aid of a digital signal processor. The seven or nine bands of such a graphic equalizer are fixed in their center frequency and quality, and are only variable in their amplification. The separate audio module of these car radio devices permits an automatic calibration of the graphic equalizer. To that end, the acoustics in the passenger compartment are measured with the aid of a microphone connected to the audio module via an analog-to-digital converter. Using a special software, the graphic equalizer is subsequently adjusted in such a way that the inadequacies of the acoustics are compensated for in the best way possible.
The use of a graphic equalizer to compensate for the inadequacies in the acoustics of a passenger compartment proves to be problematic in practice. As already mentioned, the center frequencies of the equalizer bands of a graphic equalizer are fixed. As a rule, they are spaced apart by a minimum of one octave in the case of nine bands. Thus, it is not possible to optimally compensate for narrow resonance rises, which lie between the equalizer bands, in the acoustical frequency response of the passenger compartment. Moreover, the additional audio module having the digital signal processor for implementing the graphic equalizer and for calibrating this equalizer is relatively cost-intensive.